Diamond has many outstanding properties such as high hardness and high thermal conductivity, and additionally high light transmission rate and wide bandgap energy. Diamond is therefore used widely as a material for various tools, optics, semiconductor devices or electronic components, and will become still more important in the future.
In industry, in addition to natural diamonds, artificially synthesized diamonds having a stable quality are mainly used. Currently, most of artificially synthesized single crystals of diamond are produced by synthesis in a high-temperature high-pressure environment (diamond is stable in this environment) at a temperature on the order of one thousand and several hundred degrees to two thousand and several hundred degrees and a pressure of several tens of thousands of atmospheres or more (high-temperature high-pressure method).
An ultrahigh pressure vessel in which the aforementioned high-temperature high-pressure environment is to be generated is very expensive and restricted in size. Therefore, when a single crystal diamond substrate is synthesized by the high-temperature high-pressure method, the size of the resultant single crystal diamond substrate is limited. With regard to Type Ib diamond containing nitrogen (N) as an impurity and having a yellowish color, a single crystal substrate of Type Ib diamond having a diameter of 10 mm is being manufactured by the high-temperature high-pressure method. However, the size on the order of 10 mm in diameter is considered as a substantial limit. With regard to transparent and colorless Type IIa diamond containing no impurity, a single crystal substrate of Type IIa diamond manufactured by the high-temperature high-pressure method has a diameter on the order of only several millimeters or less, except for natural Type IIa diamond.
In addition to the high-temperature high-pressure method, the vapor phase synthesis method is an established diamond synthesis method. The vapor phase synthesis method can be used to grow a diamond crystal substrate with a relatively large area having a diameter on the order of six inches (152.4 mm). Usually, a polycrystal of diamond is obtained by this method. When the diamond is used particularly as a material for an ultraprecision tool or optic required to have a smooth surface, or as a material for a semiconductor device required to have a precisely controlled impurity concentration or high carrier mobility, for example, among industrial uses of the diamond, a single crystal of diamond is used. Therefore, how to epitaxially grow a single crystal diamond substrate by the vapor phase synthesis method is being studied.
Generally, epitaxial growth includes homoepitaxial growth producing a growth layer of the same material as the material for the seed substrate, and heteroepitaxial growth producing a growth layer of a material different from the material for the seed substrate. It has been considered difficult to grow a single crystal of diamond by heteroepitaxial growth. In recent years, a free-standing diamond film having a diameter on the order of one inch (25.4 mm) has been formed, and thus growth of a single crystal diamond by heteroepitaxial growth has remarkably advanced. However, the crystal quality of the single crystal of diamond obtained by the heteroepitaxial growth is inferior to the crystal quality of a single crystal of diamond obtained by the homoepitaxial growth. It is therefore considered preferable to study how to synthesize a single crystal diamond substrate by homoepitaxial growth to thereby establish a method of manufacturing a single crystal diamond substrate having a large area.
In the case of homoepitaxial growth, high-purity diamond is grown by vapor deposition on a Type Ib diamond substrate (seed substrate) which is obtained by the high-temperature high-pressure method, for example, and thereafter the seed substrate is removed. In this way, a Type IIa diamond substrate having a larger area than a Type IIa diamond substrate obtained by the high-temperature high-pressure method can be produced. As disclosed in Japanese Patent Laying-Open No. 3-75298 (PTD 1), it has been reported that a plurality of diamond substrates or diamond crystals in the same crystal orientation can be used to integrally grow diamond thereon to obtain diamond having low angle grain boundaries only.
Problems in synthesis of a single crystal diamond substrate by homoepitaxial growth are how to remove and how to reuse the seed substrate. When a Type Ib diamond substrate or the like is used as a seed substrate to manufacture a single crystal diamond substrate, it is necessary to remove the seed substrate from the growth layer (which is to serve as a single crystal diamond substrate) by a certain method. This method may for example be a method to detach the growth layer from the seed substrate, or a method to completely remove the seed substrate. The seed substrate is formed of a single crystal of diamond and therefore expensive. In view of this, it is preferable to employ the former method. For example, a typical method is slicing with a laser beam.
However, in the case of slicing with a laser beam, a growth layer having a larger area requires a correspondingly greater thickness of the seed substrate or is accompanied by a decreased rate of success. Therefore, for a growth layer formed of a single crystal of diamond of 10 mm×10 mm for example, it is difficult to detach the growth layer from the seed substrate by slicing with a laser beam and thus the method to completely remove the seed substrate (the latter method) must be used. The method to completely remove the seed substrate may polish the seed substrate with diamond abrasive grains, may cause the seed substrate to react with an iron surface so that a part of the seed substrate which has reacted with the iron surface is removed, or may apply an ion beam to the seed substrate, for example. However, any method requires a long time to completely remove the seed substrate. Moreover, in the case of this method, the substrate (seed substrate) obtained by the high-temperature high-pressure method cannot be reused and thus the cost for manufacturing the single crystal diamond substrate cannot be reduced.
Under the above circumstances, Japanese Patent Laying-Open No. 2011-195407 (PTD 2) and Japanese Patent Laying-Open No. 2012-86988 (PTD 3) each disclose a method of detaching a diamond layer (a part to serve as a single crystal diamond substrate) by implanting ions into the diamond seed substrate to form an electrically conductive non-diamond layer and electrochemically etching away the non-diamond layer.